1. Field of the Invention
The present invention relates to a control device for a vehicle power converter, in particular, a control device for a vehicle power converter, which is connected between an AC generator-motor and a DC power source.
2. Description of the Related Art
An on-vehicle generator-motor, which is provided in an automobile, is driven to start an engine. Further, after the engine is driven, the on-vehicle generator-motor generates power. A control device for a vehicle power converter, which performs conversion between DC power and AC power, is used for the on-vehicle generator-motor.
There is a conventional vehicle power converter described in, for example, Japanese Patent No. 3396955 (hereinafter, referred to as Patent Document 1).
In general, the control device for the vehicle power converter is connected between a generator-motor and a battery. The control device for the vehicle power converter includes a plurality of switching elements and diode elements connected in parallel to the switching elements (for example, parasitic diodes associated with the switching elements). When the generator-motor operates as a motor, the control device controls ON/OFF of the switching elements to convert DC power supplied from the battery into AC power. On the other hand, when the generator-motor operates as a generator, the AC power generated by the generator-motor is rectified by the diodes or the switching elements into the DC power.
If there is a sudden load rejection due to, for example, the disconnection of a cable for connecting a positive pole of the battery and the generator-motor, which occurs during a power-generating operation for some reason, a high voltage (overvoltage) is generated at a B-terminal. At this time, the overvoltage (corresponding to a G-pulse referred in Patent Document 1) generated at the B-terminal is suppressed by bringing all the switching elements of a negative pole-side arm into conduction in a control circuit including the switching elements to short-circuit an armature winding of the generator-motor.
Patent Document 1 describes in detail means of realizing the short-circuit of the negative pole-side arm at the time of generation of the overvoltage, and the cancellation of short-circuit of the negative pole-side arm. In Patent Document 1, when the overvoltage is generated, the switching elements of the negative pole-side arm are brought into conduction to short-circuit the armature winding of the generator-motor. A condition of the cancellation of the short-circuit is described in Patent Document 1 as follows. During a predetermined period of time after the detection of the overvoltage by an abnormal voltage detector circuit section, the short-circuit of the negative pole-side arm is maintained. After the lapse of the predetermined period of time, the short-circuit is cancelled. In the technology described in Patent Document 1, after the detection of the overvoltage at the B-terminal, the switching elements of the negative pole-side arm are brought into conduction to short-circuit the armature winding of the generator-motor for the predetermined period of time. If the B-terminal is still continuously disconnected, however, the short-circuit of the negative pole-side arm is undesirably cancelled after the lapse of the predetermined period of time because a current flowing through a field winding is not limited. As a result, the power-generating operation is restarted to disadvantageously generate the overvoltage at the B-terminal again, thereby preventing electric power from being stably supplied.